Resinous coatings are commonly used to provide physical and chemical barrier protection for a wide variety of surfaces, structures, articles and the like, both during the course of production and also subsequent thereto. The extensive manufacture of printed circuit boards (PCBs) and other such electrical and electronic devices has given rise to a particularly important application for such coatings, that being in the provision of solder resists, or maskants. It is well known in the art that such resist coatings may be of either a permanent or temporary nature, removable in the latter instance by aggressive hot water washing or by use of a chemical solvent; it is also well known that the coatings may be produced from formulations that solidify or cure through a variety of mechanisms, including air-drying, thermal activation and/or photopolymerization.
The properties that a satisfactory coating formulation of this nature must exhibit will vary from case to case, but in general the following will be deemed desirable, if not essential, characteristics: fast cure, good adhesion, good resistance to deterioration under prevailing chemical and thermal conditions, satisfactory shelf-life, homogeneity, and rheological properties (e.g., smooth flow, with maintenance of shape as applied), low volatility, unobjectionable odor, non-toxicity, non-allergenicity, and non-flammability. When the coating is to be a temporary one, moreover, speed, thoroughness, and facility of removal will usually be of primary concern.
Needless to say, in those instances in which the formulation is intended specifically for use as a solder resist, the cured coating must be able to withstand the heat of the molten solder (typically applied as a wave) for at least a minimal period of time (e.g., 315.degree. Centigrade, for 15 seconds). It should in addition be resistant to solder flux and other chemicals, and should not be wetted by the solder.
Water-soluble adhesive coatings for mounting components to printed wiring boards have been provided in the past; for example Packer et al U.S. Pat. No. 4,215,025, issued Jul. 29, 1980, discloses a composition consisting of a water-soluble alcohol, water, a wetting agent, and a water-soluble, keto-group containing acid of specified structure. As far as is known, however, no formulation described in the art or heretofore available adequately satisfies the foregoing and other criteria for a solder resist, or for a similar material that is suitable for use in producing a water-soluble coating or other deposit.
Accordingly, it is the broad object of the present invention to provide a novel formulation and method for producing a water-soluble deposit upon a substrate.
More specific objects are to provide such a formulation and method from and in which the deposit produced can be readily, thoroughly and rapidly removed, using mild washing conditions and, if so desired, commercially available equipment.
Additional objects are to provide such a formulation which affords fast and complete cure, good adhesion to the underlying surface, and excellent resistance to deterioration in the chemical and thermal environment in which conventional soldering operations are carried out, while at the same time being readily applied to a substrate, providing favorable economics, and at least adequately satisfying other of the criteria enumerated herein.
A further object of the invention is to provide a method having the foregoing features and advantages which, in addition, may be particularly adapted for the production of a soldered article, especially a PCB.